Method of producing alkanesulfonic acid

ABSTRACT

The invention relates to a process for the prepartaion of alkanesulfonic acids, comprising the following steps: 
     (a) oxidation of alkylmercaptans and/or dialkyl disulfides and/or dialkyl polysulfides having from three to nine sulfur atoms with nitric acid to form alkanesulfonic acids, water, nitrogen oxides and other byproducts, 
     (b) regeneration of the nitrogen oxides obtained from step (a) with oxygen to give nitric acid and recycling of the nitric acid to step (a), 
     where steps (a) and (b) are carried out in reaction chambers separate from one another.

The invention relates to a process for the preparation of alkanesulfonicacids.

Alkanesulfonic acids are used in a number of industrial applications.Long-chain alkanesulfonic acids have, for example, surfactantproperties, while short-chain alkanesulfonic acids, such asmethanesulfonic acid, can, for example, be used as auxilary chemicalsfor the electrodeposition of noble metals such as tin or lead in the tinplating of printed circuit boards for electronics or in the preparationof tinplate.

The literature describes a number of processes for the preparation ofalkanesulfonic acids. For this purpose, alkylmercaptans or dialkyldisulfides, in particular, are used as starting materials, which areusually prepared by the reaction of hydrogen sulfide with alcohols. Theoxidation reaction of the alkylmercaptans or of the dialyl disulfides togive the corresponding alkanesulfonic acid can be achieved using avariety of oxidizing agents. For example, the oxidizing agent can behydrogen peroxide, chlorine, dimethyl sulfoxide or mixtures of dimethylsulfoxide and hydroiodic acid, and electrochemical oxidation.

WO 98/34914 describes an oxidation of mercaptans and/or dialkyldisulfides using Br₂ to give alkanesulfonic acids. The Br₂ is preferablyobtained from HBr to make handling easier. The oxidation of HBr to Br₂can be carried out with oxygen in the presence of catalytic amounts ofnitric acid or with nitric acid itself as oxidizing agent. The nitrogenoxides which form in the oxidation of HBr with nitric acid arereoxidized with oxygen to give nitric acid. In order to avoidover-oxidation of the sulfur compounds present in the process to givesulfuric acid, the oxidation of HBr to Br₂ and the oxidation of themercaptans and/or dialkyl disulfides with Br₂ can be carried out inseparate reactors.

Another method of preparing alkanesulfonic acids is the oxidation ofalkylmercaptans or dialkyl disulfides with oxygen in the presence ofnitrogen oxides or nitric acid. The oxidation with oxygen in thepresence of nitric acid is described, for example, in U.S. Pat. Nos.2,697,722 and 2,727,920.

These publications relate to the oxidation of alkylmercaptans orpolysulfides (such as dialkyl disulfides) with oxygen absorbed in nitricacid. The alkylmercaptan or the polysulfide is oxidized in stages togive the desired alkanesulfonic acid. During the oxidation, mixtures ofnitrogen monoxide, nitrogen dioxide and nitrous oxide form. The nitrogenmonoxide and the nitrogen dioxide are converted by the oxygen absorbedin the nitric acid into pure nitrogen dioxide or into nitric acid, whichin turn are available for the formation of alkanesulfonic acids. Thenitrous oxide is excluded from the system. A disadvantage of thisprocess is the high content of nitrous oxide formed which, as a“greenhouse gas” similar to halogenated methanes and ethanes, leads toecological problems and must therefore be separated off from the offgasstream in an industrial plant, which is a complex procedure.Furthermore, the offgases also comprise relatively large amounts ofnitrogen and sulfur compounds, which likewise must be removed in acomplex procedure.

The reaction temperatures for these reactions are usually in the rangebetween 25 and 70° C. However, at these temperatures complete conversionto the alkanesulfonic acid is not achieved. Thus, for example, in thereaction to give methanesulfonic acid, under these reaction conditionsthe reaction partially remains at the stage of the intermediate productS-methyl methanethiosulfonate. This intermediate is an unstable compoundwhich releases sulfur dioxide from 90° C. and decomposes spontaneouslyand extremely exothermally at 170° C.

It is therefore an object of the present invention to provide aneconomically attractive process which permits the preparation ofalkanesulfonic acids in high purity and in good yields, and suppressesvirtually completely the formation of nitrous oxide.

This object is achieved by a process for the preparation ofalkanesulfonic acids, comprising the following steps:

(a) oxidation of alkylmercaptans and/or dialkyl disulfides and/ordialkyl polysulfides having from three to nine sulfur atoms with nitricacid to form alkanesulfonic acids, water, nitrogen oxides and otherbyproducts,

(b) regeneration of the nitrogen oxides obtained from step (a) withoxygen to give nitric acid and recycling of the nitric acid to step (a).

The process according to the invention comprises carrying out the steps(a) and (b) in reaction chambers separate from one another.

Accordingly, the net reaction carried out is an oxidation of thealkylmercaptan or of the dialkyl disulfide with (atmospheric) oxygen.

The nitrogen oxides which form in step (a) are low oxidation statenitrogen compounds (NO/NO₂ mixtures), which are reoxidized in step (b)to give pure nitric acid or nitric acid containing nitrogen dioxide. Thenitric acid used in the process according to the invention can,accordingly, be pure nitric acid or nitric acid containing nitrogendioxide.

The spatial separation of the oxidation of mercaptans and/or dialkyldisulfides and/or dialkyl polysulfides having from three to nine sulfuratoms to give alkanesulfonic acid (step (a)) and the regeneration of thenitrogen oxides (step (b)) is advantageous because both reaction steps,step (a) and step (b), can be carried out separately from anotheranother under optimal reaction conditions. As a result, the formation ofnitrous oxide can be suppressed virtually completely, and it is possibleto achieve very good yields of alkanesulfonic acids.

The process according to the invention is preferably carried outcontinuously.

Step (a)

The oxidation is usually carried out at elevated temperature in order toobtain a high conversion and in order to avoid a buildup of hazardoustrace components such as methyl nitrate or S-methyl methanethiosulfateas can form during the preparation of methanesulfonic acid. In general,step (a) is carried out at reaction temperatures of from 50° C. to 150°C., preferably from 100° C. to 140° C. The operating pressure in step(a) is generally between 100 mbar and 8 bar, preferably atmosphericpressure.

The mercaptans and/or dialkyl disulfides and/or dialkyl polysulfidesused in the process according to the invention contain hydrocarbonswhich can be aliphatic or cycloaliphatic. Particularly preferably, thehydrocarbon radicals are linear or branched aliphatic hydrocarbonradicals. These preferably contain from 1 to 20, particularly preferablyfrom 1 to 14, carbon atoms. Very particularly preferably, the radicalsare methyl radicals and thus the alkylmercaptans or dialkyl disulfidesare methylmercaptan or dimethyl disulfide.

Preference is given to using dialkyl disulfides in the process accordingto the invention. The dialkyl disulfides are generally prepared fromhydrogen sulfide and methanol, although other access methods are alsoknown in the literature. Particularly preferably the dialkyl disulfidesare prepared by oxidation of alkylmercaptans with sulfur dissolved in anorganic dialkyl disulfide using an amine as catalyst. In this processthe alkylmercaptans can be used as “crude mercaptan stream”, i.e. asmercaptan stream not purified by extraction or distillation, from thereaction of alcohols with hydrogen sulfide on a suitable catalyst.

An advantage of this preparation process for dialkyl disulfides is thatthe process can be carried out at atmospheric pressure. This means thatdialkyl disulfide which is stored temporarily is not kept in apressurized container. In addition, dialkyl disulfide is astorage-stable feed material and can therefore be handled safely. Thisprocess is described in patent application Ser. No. 198 54 427.8(official file reference) which has been filed at the same time and hasthe title “Process for the preparation of dialkyl disulfides”.

The dialkyl disulfide which is preferably used is reacted to givealkanesulfonic acid and must therefore be replenished. Replenishment ofdialkyl disulfides can take place into the vapor phase of the reactionmixture in step (a) or immersed below the surface of the liquid of thereaction mixture. If the addition is into the vapor phase of thereaction mixture, intimate mixtures of dialkyl disulfides and nitrogenoxides can form, which are explosive. The dialkyl disulfides aretherefore preferably metered into the reaction mixture immersed underthe surface of the liquid. Immersion can, for example, take place in thereactor via an immersion tube or in a circulation circuit via a mixingnozzle.

The molar ration of alkylmercaptans and/or dialkyl disulfides and/ordialkyl polysulfides having from three to nine sulfur atoms to nitricacid is, for mercaptan, generally from 1:1 to 1:10, preferably from 1:2to 1:6, particularly preferably from 1:2 to 1:4. For dialkyl disulfides,the molar ratio is generally from 1:2 to 1:20, preferably from 1:3 to1:10, particularly preferably from 1:3 to 1:6.

The dialkyl polysulfides are preferably used in a mixture withmercaptans or dialkyl disulfides.

The oxidation can be carried out in one reactor or in a battery ofreactors with a high degree of back-mixing, e.g. in a stirred-tankreactor or loop reactor, or in a reactor with a low degree ofback-mixing, e.g. in a tubular flow reactor. Preference is given tocarrying out step (a) in one reactor or in a battery of reactors with ahigh degree of back-mixing. If reactors or batteries of reactors with ahigh degree of back-mixing are used, then these can, if desired, beoperated below the boiling point of the reaction mixture as pureoxidation reactors, or at the boiling point of the reaction mixture,where, during the synthesis, concentration of the reaction mixture canbe achieved simply by removing excess dilute aqueous nitric acid.

In a preferred embodiment, the oxidation part of the plant consists of abattery of two reactors with a high degree of back-mixing, e.g. twostirred-tank reactors. The temperature in the first reactor, into whichalkylmercaptan or dialkyl disulfide and nitric acid are metered, ispreferably between 50 and 140° C., particularly preferably between 80and 120° C. The second reactor, which is charged with the overflow fromthe first reactor, is preferably operated between 100 and 150° C.,particularly preferably between 130 and 150° C. with evaporation of thereactor contents. The residence times of the reaction mixture in the tworeactors can be between 10 minutes and 10 hours, preferably between 1and 3 hours.

Some of the heat of the reaction of the oxidation of the mercaptan ordialkyl disulfide is preferably dissipated via a condenser placed in theoffgas stream with condensate recycle to the reaction mixture.

If step (a) is carried out in a battery of two reactors with a highdegree of back-mixing, then the alkylmercaptan or dialkyl disulfide ordialkyl polysulfide used is largely oxidized in the first reactor, whereessentially the corresponding alkane sulfonic acid and, in a smallamount, incomplete oxidation products as well as excess nitric acid andsmall amounts of sulfuric acid form. The yield of alkane sulfonic acidin the mixture is, at this stage of the reaction, usually alreadygreater than 80%, preferably than 90%, based on the amount of mercaptanand/or dialkyl disulfide and/or dialkyl polysulfide used. In the secondreactor completion of the oxidation reaction takes place, as a result ofwhich the yield of alkanesulfonic acid is usually increased to more than90%, preferably more than 93%.

The excess nitric acid present in the discharge from reaction step (a)can be separated off distillatively by simple distillation in a mannerknown per se using water and be recycled to the oxidation of mercaptansand/or dialkyl disulfides and/or dialkyl polysulfides (step (a)) or tothe regeneration of the nitrogen oxides with oxygen to give nitric acid(step (b)). The other byproducts which form in the reaction dischargefrom step (a) can also be separated from one another distillatively, theproducts of incomplete oxidation preferably being returned to theoxidation (step a)).

In this manner, virtually all of the nitric acid is retained in thesystem, the only losses occurring due to the formation of very smallamounts of nitrous oxide and incomplete absorption in step (b). Theabsorption losses are, however, only small according to the currentposition of modern nitric acid plants.

In a preferred embodiment, the second reactor is attached to a waterseparation column operated as a stripping column. This separates offwater and nitric acid as top product, and the bottom product typicallyobtained is a colorless 98% strength alkanesulfonic acid containingapproximately 1% by weight of water and approximately 1% by weight ofsulfuric acid. Nitric acid is only present in traces of <0.2% by weight.The column is generally operated at from 20 to 1000 mbar, preferablyfrom 50 to 300 mbar and at still temperatures of generally from 130 to240° C., preferably from 150 to 200° C.

Step (b)

The regeneration of the nitrogen oxides (NO/NO₂ mixtures) is generallycarried out at low temperatures and increased pressures in order toachieve very good absorption of the regenerated nitrogen oxides NO_(x)and thus to obtain a very highly concentrated nitric acid.

For the purposes of the present invention, NO_(x) is essentially takento mean NO, NO₂, N₂O₃, N₂O₄ and N₂O₅.

The concentration of the nitric acid used in the process according tothe invention is generally from 20 to 100% by weight, preferably from 40to 70% by weight, particularly preferably from 50 to 70% by weight.Preference is given to carrying out step (b) isothermally attemperatures of from 0° C. to 60° C., particularly preferably from 0° C.to 30° C. The absolute pressures are preferably between 0.5 and 20 bar,particularly preferably between 3 and 12 bar.

The regeneration of the nitrogen oxides in step (b) is effective at thesame time as offgas washing for the sulfur compounds produced in step(a) as byproducts, meaning that the process offgas is free frommalodorous mercaptans or dialkyl disulfides or dialkyl polysulfides, andapproximately corresponds in its composition to that of current nitricacid plants. The offgas can therefore be released into the surroundingswithout additional post-treatment.

The oxygen used for the regeneration is generally atomspheric oxygen.

The reaction apparatus used is generally an absorption column. It ispreferably a cooled absorption column which corresponds to known columnsfor the preparation of nitric acid from nitrogen oxides. These can, forexample, be boiling, valve, bubble-cap, tunnel-cap columns or columnspacked with dumped or arranged packing. Cooling can take place either inthe column or in external heat exchangers.

The absorption column is generally operated at from 0 to 60° C.,preferably from 0 to 30° C., preferably isothermally. Fresh water,preferably demineralized water, is added at the top of the column, wherelean air (i.e. air depleted in oxygen) largely freed from nitrogen oxideescapes.

In the process according to the invention for the preparation ofalkanesulfonic acids, the nitric acid present in the reaction dischargein step (a) is, accordingly, preferably returned, following removal fromthe reaction discharge, to step (a) or step (b), and the products ofincomplete oxidation which are likewise present are, after removal,returned to step (a).

The already very pure alkanesulfonic acid obtained in the processaccording to the invention can be purified in a downstream vacuumdistillation column, which generally operates at head pressures of from0.1 to 20 mbar, preferably from 2 to 10 mbar. In this case, impuritieswhich occur in traces are separated off at the top or at the bottom ofthe column. The actual alkanesulfonic acid is usually obtained in asidestream takeoff. The resulting alkanesulfonic acid is colorless andgenerally has a purity of >99%, preferably of >99.5% with a sulfuricacid content of >50 ppm. Methanesulfonic acid obtained in this way issuitable, for example, for use in electrochemical baths.

Very particularly preferably, methanesulfonic acid is prepared in theprocess according to the invention by oxidation of dimethyl disulfide.The methanesulfonic acid obtained after purification (vacuumdistillation) generally has a purity of >99% and is colorless. Thesulfuric acid contents are generally less than 50 ppm. Such amethanesulfonic acid is particularly suitable for use in electrochemicalbaths.

In the accompanying drawings, FIG. 1 shows diagram-matically the processaccording to the invention. Here,

R1 is reactor 1 in which step (a) is carried out

R2 is reactor 2 in which step (b) is carried out

RSH/R—S—S—R is mercaptan used and/or dialkyl disulfide used

HNO₃ is nitric acid used

r-HNO₃ is nitric acid recycled from step (b) into step (a)

NO/NO₂ are low oxidation state nitrogen compounds (NO/NO₂ mixtures)

H₂O is water

“O₂” is atmospheric oxygen

X is offgas

P is the reaction discharge, comprising the reaction product

The example below additionally illustrates the invention.

EXAMPLE

Experimental Set-up

The attached plant diagram (FIG. 2) shows the experimental set-up.

Apparatus

A 1^(st) oxidation reactor (stirred-tank reactor)

B 2^(nd) oxidation reactor (stirred-tank reactor)

C Condenser

D Condenser

E Plate column containing 44 bubble-cap plates for the regeneration ofnitric acid

F Buffer container for nitric acid

G 1^(st) vacuum rectification column with glass ring packing

H Bottom product heat exchanger

I Condenser with reflux divider

J 2^(nd) vacuum rectification column with arranged packing

K Bottom product heat exchanger

L Condenser with reflux divider

Streams

1 Feed of pure dimethyl disulfide

2 Nitric acid feed

3 Air feed

4 Feed of deionized water

5 Exhaust air

6 Low-boiling component outflow

7 Methanesulfonic acid outflow

8 High-boiling components outflow

9 Outflow from the first to the second oxidation reactor

10 Offgas from the first oxidation reactor

11 Offgas from the second oxidation reactor

12 Condensate stream from the second oxidation reactor to the nitricacid regeneration

13 Outflow from the second oxidation reactor to the first vacuumrectification column

14 Condensate stream from the first vacuum rectification column for thenitric acid regeneration

15 Bottom product discharge from the first to the second vacuumrectification column

Experimental Details

The reactor A is charged continuously, with stirring, via 1 with puredimethyl disulfide (>98%) and from F with 45 to 50% strength nitric acidin the DMDS (dimethyldisulfide): HNO₃ molar ratio of 1:5. Thedimethyldisulfide is introduced beneath the surface. The temperature inthe reactor A is 100° C. The residence time in reactor A, calculated asa quotient of the liquid volume in reactor A, divided by the liquidstream 9 which is continuously leaving reactor A, is about 2.2 h.

The liquid stream 9 which is continuously leaving reactor A consists ofabout 32% of methanesulfonic acid, 11% of nitric acid, 0.6% of S-methylmethanethiosulfonate and 56% of water and is fed to reactor B. Thetemperature in reactor B is 130° C. The residence time in reactor B,calculated as a quotient of the liquid volume in reactor B, divided bythe liquid stream 13 which is continuously leaving reactor B, is about2.2 h. The liquid stream 13 which is continuously leaving reactor Bconsists of about 55% of methanesulfonic acid, 10% of nitric acid, <0.2%of S-methyl methanethiosulfonate and 35% of water and is passed to thevacuum rectification column G just below the top of the column. Thecrude yield of methanesulfonic acid in stream 9 is >95%.

The vacuum rectification column G operates at a head pressure of from 95to 100 mbar (absolute) and a still temperature of 180 to 190° C.

The bottom product 15 leaving the vacuum rectification column G consistsof about 98% of methanesulfonic acid, about 1% of water and about 1% ofsulfuric acid and is fed to vacuum rectification column J.

Vacuum rectification column J operates at a head pressure of from about5 to 10 mbar (absolute) and a still temperature of from about 180 to190° C.

The side take-off stream 7 leaving the vacuum rectification column Jconsists of >99% strength methanesulfonic acid having a sulfuric acidcontent of <50 ppm. The total yield of methanesulfonic acid afterdistillation is >90%.

The head take-off stream 6 leaving the vacuum ectification column Jconsists of water, methanesulfonic acid, methyl methanesulfonate andother low-boiling components. The bottom product take-off stream 8leaving the vacuum rectification column J consists of sulfuric acid,methanesulfonic acid and other high-boiling components.

The plate column for the nitric acid regeneration E operates at normalpressure and at temperatures of 20 to 45° C.

The ionized water is fed via feed 4 to the plate column for theregeneration of nitric acid E.

The air introduced into the plate column for the regeneration of nitricacid E via stream 3 for the reoxidation of the nitrogen oxides leavesthe column at the top exit 5 with a reduced oxygen content (7 to 13% byvolume).

The NO_(x)-containing offgas stream 10 formed in reactor A and freedfrom condensable components in condenser C comprises NO and NO₂ and ispassed to the plate column for the regeneration of nitric acid E.

The NO_(x)-containing offgas stream 11 formed in reactor B and freedfrom condensable components in condenser D comprises NO and NO₂ and ispassed to the plate column for the regeneration of nitric acid E.

The condensate 12 consists of about 7% strength nitric acid and ispassed to the plate column for the regeneration of nitric acid E to aplate having a similar plate concentration of nitric acid.

The condensate 14 consists of about 23% strength nitric acid and is fedto the plate column for the regeneration of nitric acid E to a platehaving a similar plate concentration of nitric acid.

The bottom product outflow from the plate column for the regeneration ofnitric acid E to the nitric acid buffer container F consists of about 45to 50% strength nitric acid and is fed to the reactor A.

Nitric acid losses are replaced by topping up the required amounts of 50to 65% strength nitric acid via stream 2 into the nitric acid buffercontainer F.

We claim:
 1. A process for the preparation of alkanesulfonic acids,comprising the following steps: (a) oxidation of alkylmercaptans and/ordialkyl disulfides and/or dialkyl polysulfides having from three to ninesulfur atoms with nitric acid to form alkanesulfonic acids, water,nitrogen oxides and other byproducts, (b) regeneration of the nitrogenoxides obtained from step (a) with oxygen to give nitric acid andrecycling of the nitric acid to step (a), which comprises carrying outsteps (a) and (b) in reaction chambers separate from one another.
 2. Aprocess as claimed in claim 1, wherein the process is carried outcontinuously.
 3. A process as claimed in claim 1, wherein step (a) iscarried out at reaction temperatures of from 50 to 150° C. and at anoperating pressure of from 100 mbar to 8 bar.
 4. A process as claimed inclaim 1, wherein the alkyl mercaptans or the dialkyl disulfides or thedialkyl polysulfides contain hydrocarbon radicals having from 1 to 20carbon atoms.
 5. A process as claimed in claim 1, wherein dialkyldisulfides are oxidized in step (a).
 6. A process as claimed in claim 5,wherein dimethyl disulfide is oxidized.
 7. A process as claimed in claim1, wherein some of the heat of the reaction of the oxidation of themercaptan or of the dialkyl disulfide or of the dialkyl polysulfide isdissipated by a condenser placed in the offgas stream with condensaterecycling.
 8. A process as claimed in claim 1, wherein step (b) iscarried out at from 0 to 60° C. and at an operating pressure of from 0.5to 20 bar.
 9. A process as claimed in claim 1, wherein step (b) iscarried out in a cooled absorption column.
 10. A process as claimed inclaim 1, wherein the nitric acid present in the reaction discharge instep (a) is, after having been removed from the reaction mixture,returned to step (a) or step (b), and the products of incompleteoxidation which are likewise present are, after they have been removed,returned to step (a), and the alkanesulfonic acid is separated off.